JARS v51n4 - Testing Rooting Hormones

Testing Rooting Hormones
Marc Colombel
Fouesnant, France

Like many rhododendron lovers, I am very interested in rhododendron propagation. Rooting cuttings is one way to propagate them, but even if one knows the technique using wounds and bottom heat there is still a question: which hormone is the best one? This question is so important that regularly there are articles about it.
One should remember that in order to compare two, or several, hormones, it is necessary that all elements are the same. Nevertheless, the following problems do arise:
1.  Cuttings should be as cars at the end of an assembly line - that is to say, true copies. This is never the case. Their thickness is not the same, nor is their hardiness or their reserves of water because not only were they cut off from different parts of one or several rhododendrons but mainly because nature is not a factory.
2.  The temperature should be the same for all the cuttings. It is difficult to imagine that this condition is met because the cuttings are not exactly at the same height above the heat source, and even if they were the temperature is not exactly the same in all the heated surface.
Because of these problems I decided to test my own method which consists of applying two different hormones on each cutting. I make two opposite wounds of equal length at the base of the cutting. Then I apply one hormone on a wound with a cotton bud and another one on the second wound with another cotton bud. I put a mark on the base with a pencil, in general three short lines to remind me that when I have these three lines in front of me, the NAA (naphthalene acetic acid) is on the left and the IBA (indole butyric acid) is on the right. (This is only an example, of course.) That is all.
The advantages of this method of testing hormones are:
1.  You can use as many cuttings as you want. The comparison between both hormones is accurate even if all the cuttings are different.
2.  You can insert the cuttings without taking special care that both cuttings will be strictly at the same temperature.

Cuttings showing the use of the hormone 
naphthalene acetic acid on the left and indole butyric acid on the right.
Cuttings of R. 'Colonel Rogers' ( R. falconeri x R. niveum ) showing
the use of the hormone naphthalene acetic acid on the
left and indole butyric acid on the right.
Photo by Marc Colombel

After a few weeks you can see several roots on the right side and no roots on the left side, proving that IBA is better than NAA (in a 0.2% concentration). I do not test other concentrations.